Reloadable disks are conventionally used for many purposes. A disk typically includes a disk-type transparent substrate, and a thin recording layer on one surface of the substrate. Data is repeatedly recorded and deleted by reversibly changing the optical density of the thin recording layer. The thin recording layer of the disk is crystallized in advance, and is then heated, melted and quenched by irradiating the thin recording layer with about a 1 .mu.m laser beam and changing the intensity of the beam. As a result, the layer becomes amorphous and records information. In addition, the amorphous recording thin layer is crystallized by raising the temperature of the layer in a range between the crystallization temperature and the melting point and then annealing the layer, thus deleting information.
When a resin substrate, for example, is used for the disk-thin recording layer is formed directly on the substrate, the substrate is heated to a high temperature in a minute section of about 1 .mu.m by recording and deleting and is thus deformed. Therefore, in general, a dielectric layer is formed as a heat insulating layer between the substrate and the thin recording layer and between the thin recording layer and a layer protecting the thin recording layer (protective layer), thus preventing thermal deformation of the substrate. Since the temperature rise, quenching and annealing properties of the thin recording layer vary due to the heat conduction properties of the protective layer, recording and deleting characteristics of the disk are improved by selecting preferable materials and layer composition. Furthermore, the recording and deleting characteristics of the disk are improved by forming a reflecting layer on the surface of the dielectric layer facing the protective layer and utilizing the multiple interference of a laser beam. This four-layer disk structure is generally well known.
A rapid cooling disk structure is proposed for the four-layer reloadable disk. In the rapid cooling disk structure, the dielectric layer between the thin recording layer and the reflecting layer (identified as a second dielectric layer hereinafter) is thinned so that heat generated in the thin recording layer during recording and deleting is quickly released to the reflecting layer. The rapid cooling disk structure has an advantage in that the outer limits of the erase rate and power improve since the temperature of the dielectric layers on both sides of the thin recording layer rises by widely dispersing the heat from the thin recording layer. It is also advantageous in making the thin recording layer amorphous since the layer is cooled quickly in this structure. Japanese Patent Application No. Sho 63-207040 (Published Unexamined (Laid-open) Japanese Patent Application No. Hei 02-056746) discloses a rapid cooling disk structure. In this application, the second dielectric layer is thinner than the dielectric layer between the disk substrate and the thin recording layer (identified as a first dielectric layer hereinafter) and is 30 nm thick or less.
However, when the thickness of the second dielectric layer is 30 nm or less, the recording and deleting sensitivities of the disk decline, so that an expensive high power semiconductor laser has to be used. In addition, the first and the second dielectric layers are under thermal stress, expanding and shrinking due to rapid heating at 400.degree. C. or higher and cooling for repeated recording and deleting. Although the second dielectric layer has a smaller heat load than the first dielectric layer, the layer repeatedly receives thermal stress. The layer should thus have a proper thickness. The thickness of the second dielectric layer has to be selected in consideration not only of the heat conduction but also of optical properties.